1. Field of the Invention
The present invention relates to a photometric apparatus for use in a camera and capable of obtaining brightness information, and, more particularly, to a photometric apparatus,arranged to obtain brightness information from an output from an image sensor for use in detecting a focal point.
2. Related Background Art
The photometry of a camera is performed by a variety of methods, for example, by an average photometry in which the overall frame to be photographed is subjected to photometry, a central weighted photometry in which the central portion of the frame is in main subjected to the photometry and a spot photometry in which a very narrow portion at the central portion of the frame is subjected to the photometry.
Recently, auto-focusing has been realized in cameras, the auto-focuing being realized by converting beams at the central portion of the frame to be photographed into an electric signal by an image sensor array consisting of a plurality of photo-sensor pixels so that the focal detection is conducted in response to the thus formed electric signal. A structure capable of performing the spot photometry has been known, the structure being arranged in such a manner that the electric signal generated by the image sensor array is also used as brightness information.
However, the image sensor array for detecting the focal point is of a charge stored type such as a CCD image sensor. Therefore, the following problem arises:
In general, in a photometric apparatus, the charge generated by a photo-diode is received in the form of the value of an electric current. Therefore, brightness information at a certain time can be instantaneously obtained. Therefore, the light source which is illuminating a subject prevents change in brightness information in a case where the brightness of the light source for illuminating the subject is cyclically changed. Therefore, even if the structure is arranged in such a manner that brightness information for several times is averaged, the processing can be completed in a short time.
However, if the charge stored type image sensor such as a CCD image sensor is used for detecting the focal point, the time for storing the charge, that is, the storage time and transfer time for transmitting the outputs from a plurality of photo-sensors by a shift register in a time sequential manner must be taken in order to obtain an output signal from the sensor. Therefore, when the structure is arranged in such a manner that the brightness information for several times is averaged for the purpose of preventing the change in the brightness information due to light having cyclic characteristics such as a fluorescent lamp, an excessively long time is required.